1) Field of the Invention
The present invention relates to a measurement method of measuring a characteristic of a mirror system having a mirror plane variable to arrange an inclination, and more particularly to a technique suitable for use in measurements of a deflection characteristic of a mirror system based on reflected light from a mirror plane.
2) Description of the Related Art
In recent years, for example, in communication fields using optical fibers, an optical switch which enables a path of an optical signal to be switched in a state of light has been employed in a case in which an optical signal is switched form one optical fiber network, in which it currently flows, to a different optical fiber network, or in other cases. As this optical switch, there has commonly been used a mirror system which includes a mirror made to change a propagation direction of an optical signal by reflecting the optical signal and controls a deflection angle (inclination) of a mirror plane (surface) of this mirror to realize the three-dimensional switching of an optical signal. In addition to the optical switch, this mirror system has also been employed for an apparatus in which mirrors are disposed in the form of an array so as to use the mirror array for the scanning by incident light.
As the mirror systems, for example, there have been known an MEMS (Micro Electro Mechanical Systems) mirror designed to control a deflection angle of a mirror plane by use of an electrostatic force as shown in FIG. 5 of the following Patent Document 1 and a galvanomirror in which a mirror plane is mounted on a shaft of a motor so that the deflection angle of the mirror plane mounted on the motor shaft is controlled by driving the motor by use of an electromagnetic force.
FIG. 16 illustratively shows an example of an arrangement of an MEMS mirror. For example, as shown in FIG. 16, the MEMS (Micro Electro Mechanical Systems) mirror, generally designated at reference numeral 70, is made up of a mirror plane 71, an inner frame 72, an outer frame 73, first torsion bar springs 74, 74 and second torsion bar springs 75, 75. The first torsion bar springs 74 and 74 are disposed along an X-axis direction so as to perpendicularly intersect a pair of sides of the rectangular mirror plane 71, confronting each other, at central positions of the pair of sides thereof, respectively. Moreover, in the MEMS mirror 70, the second torsion bar springs 75 and 75 are disposed along a Y-axis direction perpendicular to the X-axis direction so as to perpendicularly intersect a pair of sides of the rectangular inner frame 72, confronting each other, at central positions of the pair of sides thereof, respectively. Still moreover, the mirror plane 71 is attached through the first torsion bar springs 74 and 74 to the inner frame 72 so as to be rotatable around the X axis, while the inner frame 72, together with the mirror plane 71, is attached through the second torsion bar springs 75 and 75 to the outer frame 73 so as to be rotatable around the Y axis.
In addition, the MEMS mirror 70 includes a drive circuit (not shown) which generates an electrostatic force in response to the input of a voltage, and the deflection angle of the mirror plane 71 is changeable without restriction by means of tortional functions of the first torsion bar springs 74, 74 or the second torsion bar springs 75, 75 which correspond to this electrostatic force.
The mirror system such as the above-mentioned MEMS mirror 70 is designed such that its deflection angle is controlled by receiving an input of a voltage, and an individual difference can occur among deflection characteristics (maximum deflection angles, deflection angles when a predetermined voltage is inputted, deflection velocities and resonance points when an inputted voltage is changed by a predetermined oscillation frequency, and others). For example, even when the same voltages are inputted to a plurality of mirror systems having the same arrangements, a difference may arise in deflection angle or resonance point and in deflection velocity relative to the inputted voltage and, hence, there is a need to prepare a processing at the time of manufacturing to measure a deflection characteristic of the mirror system for, on the basis of the measurement result, correcting a voltage set value which can operate the mirror system.
In general, the measurement of the deflection characteristic of the mirror system is made on the basis of reflected light reflected from a mirror plane (which will hereinafter be referred to simply as reflected light) and, as conventional techniques, there have been known a method (PSD method) of measuring the intensity and position of reflected light by using a PSD (Position Sensitive Device) element, a method (laser Doppler oscillation-system method) of measuring the interference of reflected light by using a laser Doppler oscillation system, and other methods.
FIGS. 17 and 18 are illustrative views showing examples of arrangements of conventional measurement apparatuses. For example, as shown in FIG. 17, a measurement apparatus 80 based on the PSD method is composed of a measurement light source 82 made to emit measurement light 81 onto a mirror plane 71 and a PSD element 84 made to receive reflected light 83 produced by the reflection of the measurement light 81, emitted from the measurement light source 82, on the mirror plane 71. When the PSD element 84 receives the reflected light 83, the measurement result including the intensity of the reflected light 83 and the incidence position on the PSD element is outputted as a voltage (analog signal) to evaluation equipment (not shown) such as a computer. Moreover, the evaluation on the deflection characteristic of the mirror system is made on the basis of the inputted voltage (voltage value, oscillation frequency, or the like) inputted to the mirror system and the measurement result of the reflected light on the PSD element 84.
[Patent Document 1]                Japanese Patent Laid-Open No. 2005-283932        
Meanwhile, for example, for the measurement of a mirror system at the manufacturing, there is a case in which the characteristic correction is dynamically made using a result of the measurement and, in this case, there is a need to accurately measure the deflection characteristic of the mirror system within a short period of time.
There is a problem which arises with the above-mentioned PSD method, however, in that, since the PSD element 84 is designed to output the positional information on the reflected light 83 as an analog signal, the reliability (stability) of the positional information suffers degradation, thereby making it difficult to accomplish the measurement with high accuracy. In addition, in a case in which, for example, as shown in FIG. 18, a protection cover glass 85 is located outside the mirror system 70, the PSD element 84 can receive not only the reflected light 83 from the mirror plane 71 but also the reflected lights 86a, 86b and 86c from a front surface 85a and rear surface 85b of the cover glass 85. In the case of the PSD element 84, difficulty is encountered in achieving the accurate measurement in such a case of the simultaneous reception of a plurality of inputted lights.
Moreover, in a case in which the reflected light 83 moves at a high speed due to high-speed oscillation (deflection) of the mirror plane 71, difficulty is experienced in accurately reading out the movement quantity of the reflected light 83 due to the restriction on the response performance of the PSD element 84.
Furthermore, in the case of the employment of the laser Doppler oscillation-system method, there is a need to measure the interference between the reflected light 83 and the light appearing due to the reflection of the reflected light 83, which encounters complicated condition setting such as location of a measurement apparatus and suffers an increase in cost of the measurement apparatus.